Heparin is a naturally occurring biomolecule that is used for many medical applications. One application takes advantage of heparin's binding to the biomolecule antithrombin III (AT III). Heparin is introduced into a patient's blood, where it binds ATIII and thereby helps prevent unwanted blood clotting. Heparin binds to AT III by interacting with specific heparin-binding sites on ATIII. Heparin's negatively charged sulfate and sulfonate groups play an important role in this binding.
Many biomolecules have heparin-binding sites but heparin binds them only weakly or with little specificity. Without specificity for a target, heparin given to a patient is taken up by other biomolecules and prevented from reaching its target. And if it does reach its target, a weak bind may cause it to have little effect. Heparin, in fact, has many limitations concerning the specificity, speed, and strength of its interactions with other molecules.
Combinatorial chemistry is a technology that involves making many chemicals and screening them. The screening test is used to test the chemicals to determine which ones have a useful chemical property with regards to a given target. Combinatorial chemistry has been successfully used to make many drugs.